The term “rotorcraft” is used to designate an aircraft in which lift is provided in full or in part by one or more propellers of substantially vertical axis and of large diameter, commonly known as rotors or as rotary wings.
In the category of rotorcraft, various distinct types are known, however, in addition to a main rotor, a hybrid helicopter in accordance with the invention has at least one propulsive propeller, and preferably two variable-pitch propulsive propellers forming parts of two propulsion units on the left and on the right of the hybrid helicopter.
A conventional helicopter has at least one main rotor that, under drive from a suitable power plant, serves both to provide it with lift and with propulsion. A helicopter is capable of hovering, i.e. remaining at a stationary point in three dimensions, it can take off and land vertically, and it can move in any direction (forwards-rearwards-sideways-up-down).
The driving power of a conventional helicopter (not having any propulsive propellers) is generally regulated by means of a control member/module that adapts the power delivered by the power plant to the power required by the dynamic assemblies (rotor(s) and accessories), so as to maintain the speed of rotation of the main rotor(s) and of the power transmission system at its setpoint value.
In an aircraft that is propelled by one or more variable-pitch propellers, power regulation generally includes a regulation member/module (in general of the hydromechanical type) that adapts the pitch of the propulsive propeller(s) so as to consume all of the available power that results from how the pilot has set a throttle (or thrust) control member or lever.
Those two adjustments cannot be juxtaposed for regulating the power of rotorcraft fitted with one or more propulsive propellers, since those modes of regulation are antagonistic. The member for adapting power when regulating a helicopter in conventional manner opposes any transient variation in the speed of the power transmission system of the kind that results from varying thrust from the or each propeller.
Furthermore, for a rotorcraft fitted with one or more propulsive propellers, regulating the propulsive propeller(s) by the pilot directly controlling propulsive propeller pitch variations could give rise to damage thereto, as a result of sudden changes in the engine torque transmitted to the propeller(s).
For example, documents U.S. Pat. No. 4,488,851 and U.S. Pat. No. 4,514,142 disclose a helicopter having a main lift rotor and also having a propulsive tail rotor. A control system enables the pilot to limit the power consumed by the propulsive tail rotor to the advantage of the power required by the main lift rotor. Such control carried out by a pilot does not make piloting operations any easier.
Document FR 2 916 421 discloses a control system for a rotorcraft having a rotor, at least one variable-pitch propulsive propeller, and an engine driving the rotor and the propeller. The system includes a member for generating a propeller pitch setpoint as a function of a thrust variation control order, a member for generating a drive speed setpoint for the rotor and for the propeller as a function of the travel speed of the rotorcraft, and a member for generating an engine speed setpoint as a function of the thrust variation control order, of the drive speed setpoint, and of a collective pitch control order for the rotor.
In a hybrid helicopter having variable-pitch propellers, the pilot must both limit upward variation of the collective pitch and consequently power transmission to the rotor(s) from the power plant via the transmission members so as to avoid exceeding mechanical or thermal limits of those elements, and also, and for the same reasons, limit the propeller thrust control, i.e. the propeller pitch control, as explained below.
Furthermore, since the power plant of a hybrid helicopter is constituted by one or more turbine engines, the speeds of rotation at the outlet(s) from the turbine engine(s), of the propeller(s), of the rotor(s), and of the mechanical interconnection system are mutually proportional, with the proportionality ratio being constant regardless of the flight configuration of the hybrid helicopter under normal conditions of operation of the integrated drive train.
It can thus be understood that if the hybrid helicopter is fitted with only one turbine engine, it is that engine that drives the rotor(s) and the propeller(s) via the mechanical interconnection system. However, if the helicopter is fitted with two or more turbine engines, then the rotor(s) and the propeller(s) are driven in rotation by said turbine engines via the interconnection mechanical system.
In other words, the drive train operates without any variable ratio between the speeds of rotation of the turbine engine(s), the propeller(s), the rotor(s), and the mechanical interconnection system.
Consequently, the rotor(s) advantageously continue(s) to be driven in rotation by the turbine engine(s), and continue(s) to develop lift whatever the configuration of the aircraft.
More precisely, the rotor(s) is/are of the kind designed to provide all of the lift of the hybrid helicopter during stages of taking off, landing, and hovering, and some of the lift during cruising flight, with an auxiliary wing then contributing a portion of the lift for supporting said hybrid helicopter.
Thus, the rotor(s) deliver(s) only part of the lift for the hybrid helicopter in cruising flight and possibly also a small contribution to propulsive or traction forces (in a helicopter), but no contribution to drag (in an autogyro). These operating conditions thus lead to a reduction in the delivery of power that is dedicated to traction provided by the rotor(s). A small contribution to propulsive forces is provided by the rotor(s) being tilted towards the front of the aircraft by a small amount only. That degrades the fineness of the rotor(s) very little and is consequently more advantageous in terms of power balance than a demand for additional thrust exerted by the propeller(s).
Advantageously, the wing is made up of two half-wings, each half-wing being on a respective side of the fuselage. The half-wings may together form a high wing in which case they preferably present a negative dihedral angle. Nevertheless, they could equally well be constituted by a low wing in which case they would preferably have a positive dihedral angle, or indeed an intermediate wing of arbitrary dihedral angle. The shape of the half-wings in plan view could correspond to half-wings that are rectangular, tapering, or back-swept, etc.
Thus, by controlling the thrust from or the mean pitch (also known as the half-sum pitch) of the propulsive propellers, the pilot might accidentally give rise to a drop in the speed of the main rotor(s), might exceed the velocity never exceeded VNE (maximum authorized air speed), or might exceed mechanical strength limits, or might exceed thermal limits of the propulsive unit(s), of the turbine engine(s), or of the propeller(s).